This invention relates to a semiconductor device and a method for manufacturing the same, more specifically to a semiconductor device including a semiconductor substrate with controlled defect density and a method for manufacturing the same.
Bipolar devices are typical semiconductor devices in which an epitaxial layer is formed on a semiconductor substrate. A bipolar device may be manufactured in processes as shown in FIGS. 1A to 1C. First, the specular-ground surface of a P-type single-crystal silicon substrate 1 shown in FIG. 1A is oxidized to form an oxide layer 2, and a predetermined portion of the oxide layer 2 is removed by photolithography, as shown in FIG. 1B. Then, antimony is selectively diffused in the exposed surface of the silicon substrate to a given depth at a temperature of e.g. 1,250.degree. C. to form an N-type region 3. Subsequently, the oxide layer 2 is removed by using hydro-fluoric acid, and then an epitaxial layer 4 is grown over the substrate 1 by hydrogen reduction of silicon tetrachloride, as shown in FIG. 1C.
In the epitaxial layer 4 formed in the aforementioned manner, there usually exist defect called stacking faults 5. The density of the stacking faults 5 varies within a wide range of 3/cm.sup.2 to 1,000/cm.sup.2. Generally, if contaminations such as Fe and Cu exist in the single-crystal substrate 1, or if such contaminations are induced during an epitaxial growth step including a diffusion step, then the density of stacking faults 5 will increase with the contamination which will be nuclei 7 of the stacking faults. The density of these stacking faults should be minimized since they may exert a great influence on the manufacturing yield and electrical properties of the semiconductor device.
In order to prevent the occurrence of the stacking faults, there are proposed several cleaning methods. According to one of these methods, the purity of the atmosphere during the epitaxial growth step including the diffusion step is improved. According to another method, the oxygen concentration of the substrate 1 is increased. Although the former method or the improvement of the purity of the atmosphere during the manufacturing step is attempted frequently, it is very difficult to achieve a marked improvement at once. The latter method or the attempt to prevent the occurrence of stacking faults by increasing the oxygen concentration of the substrate, which appears in J. Electrochem. Soc. 125, 1151 (1978); L. E. Katz and D. W. Hill, can be regarded as highly effective. This method, however, has the following two problems. First, if the number of revolutions is increased to increase the oxygen concentration in growing the single crystal by Czochralski method, then dislocation will be liable due to the vibration of molten silicon surface, etc. Secondly, the oxygen concentration distribution in the single crystal will not be uniform along the longitudinal direction (direction of growth). Namely, the oxygen segregation coefficient k at the solid-liquid interface of silicon is 1.2-higher than 1. Therefore, the oxygen concentration is high on the seed side (head side) of the single crystal, though it is low on the opposite side. Table 1 shows an example of measurement of the oxygen concentration on the head and tail sides of the single crystal. This measurement is made by infrared absorption method, and the oxygen concentration is obtained as .DELTA..alpha..times.2.6.times.10.sup.7 /cm.sup.3 (.DELTA..alpha. is increment of infrared absorption coefficient attributable to oxygen).
TABLE 1 ______________________________________ oxygen concentration ( /cm.sup.3) Sample No. head side tail side ______________________________________ 1 11.4 .times. 10.sup.17 7.8 .times. 10.sup.17 2 11.1 .times. 10.sup.17 8.3 .times. 10.sup.17 3 11.6 .times. 10.sup.17 8.6 .times. 10.sup.17 ______________________________________
As is evident from Table 1, the single crystal can have high oxygen concentration only at the head-side portion, so that the occurrence of stacking faults can be prevented only at such portion. According to an experiment conducted by the inventors hereof, the portion of the single crystal exhibiting high oxygen concentration and hence suffering less stacking faults, which was found only on the head side, proved to account for 20% of the whole single crystal in volumne.